DYNAMIC NMR STUDY OF THE ELECTRONIC STATE OF HETEROAROMATICS VI. COMPETING C,N AND C,C BARRIERS TO ROTATION OF PUSH-PULL ETHYLENES WITH AROMATIC TRANSITION STATES IN THE 1,3- THIAZINE SERIES

Abstract

The barriers to rotation about the exocyclic partial C,N and C,C double bonds of a series of substituted 4-phenyl-2-NR₂-6-methylendH-l,3-thiazines were studied and found to be strongly dependent on structural influences. The C.N barrier is determined by the degree of mesomerism in the ground state of the restricted rotation, the C,C barrier, however, is dependent on the capacity of the system to stabilize the negative charge of the dipolar transition state exocyclically and the positive charge endocyclically via the aromatic thiazinium 6π electron system. Hereby the mesomeric effect of the NR, group is of special influence. ¹H as well as ¹³C chemical shifts parallel obtained conclusions. The π-electron acceptors and analogues of the latter are of the same influence on the C.N barrier to rotation in line with the pseudochalcogeno analogy principle.

Key Words:

• Rotational barriers
• Restricted C,N and C,C rotations
• Transition and ground state stability
• Pseudochalcogeno analogy principle.